Water Fasting Side Effects

Water fasting has become trendy within the past few years as a result of being promoted by prominent public figures, YouTubers, medical doctors, and scientists as an enhancer of health and adjunct treatment for various medical conditions.

The purpose of this article is NOT to scare people away from water fasting.

The purpose of this article is to highlight common water fasting side effects – and explain why each might occur.

Disclaimer: If you have any questions about water fasting and/or specific side effects that you’re experiencing – I recommend consulting a medical doctor (MD) – preferably one familiar with water fasting (and the medical literature).

What is water fasting?

Water fasting is defined as a period of caloric abstinence (i.e. zero food) – while consuming water “as needed” (i.e. PRN) to prevent dehydration.

Clear glass with water pouring inside.

agua, agua, agua

Since there’s no formal (medical or scientific) definition for “water fasting” – one could argue that any duration (e.g. 5 minutes, 5 hours, or 15 hours) with: (1) zero calorie (i.e. food) intake AND (2) water consumption as needed – is a “water fast.”

However, most proponents of water fasting and laypersons agree that, in order to meet criteria for a “water fast” – one’s caloric abstinence (i.e. fasting period) must be greater than OR equal to 24 hours.

Why? Although there’s no scientific consensus of a minimum time threshold to be considered a “water fast” – most people reference “water fasts” as extended/prolonged fasts (usually in excess of 24 hours).

Technically speaking, “water fasting” can co-exist with things like “intermittent fasting” (IF) and “time-restricted eating” (TRE).

That said, the general public tends to regard “intermittent fasts” (IF) fasts for brief/short periods (usually less than 24 hours).

The duration of caloric abstinence required to be considered an official “water fast” varies among practitioners, however, some fasting experts believe the time requirement should be set at 72 hours.

Why? Because fasting for at least 72 hours usually ensures that metabolism will have completely shifted from utilizing glucose (via glycolysis/gluconeogenesis) to utilizing ketones for energy (such that one enters “ketosis”).

Many health benefits associated with water fasting are hypothesized to be attained from: (1) reaching AND (2) maintaining (for adequate duration) – a state of ketosis with low glucose.

If one’s purpose for water fasting is to prevent and/or adjunctively treat medical conditions in which high glucose and aerobic fermentation are problematic (e.g. various: cancers, neurodegenerative disorders, traumatic brain injuries, inflammatory diseases, epilepsy) – then it may be necessary to remain within a specific glucose-ketone index (GKI) range for a specific (sometimes lengthy) predetermined duration.

(The glucose-ketone index (GKI) is calculated as a ratio of glucose (mg/dL) to ketones (mM/L) – while controlling for respective molecular weights).

Thomas Seyfried, a prominent cancer researcher at Boston College, suggests that water fasting once or twice per year for 5-to-7-days (120-168 hours) may be an effective prophylactic and/or adjunct treatment for certain cancers.

Why? Because various cancers rely upon aerobic fermentation of glucose for sustenance and/or growth.

By water fasting for 5-to-7 days, one’s GKI (glucose-ketone index) reaches 1.0 for a sufficient duration to restrict the fuel (glucose fermentation) of cancer and pre-cancer cells – thereby counteracting their growth and/or inducing apoptosis (programmed cellular death).

Regardless of the specific definition and/or criteria for “water fasting” to which you subscribe – side effects regularly occur during the fasting process.

Water fasting side effects

Included below is a list of common side effects that emerge while water fasting.

Understand that the specific side effects that you experience – and their severities (i.e. magnitudes) – will be individualized.

In other words, the side effects that you endure during your water fast might differ from those reported by a friend, your favorite YouTuber, or medical case reports.

If you have any medical condition(s), a history of serious medical events, are using medications, and/or are at a low body weight – it is never recommended to engage in water fasting without prior authorization from a medical doctor.

Once again, if you have concerns or questions about any specific side effect of water fasting – consult a medical doctor.

Orthostatic hypotension

Also known as postural hypotension, orthostatic hypotension refers to a sudden drop in blood pressure while transitioning from (1) seated positions OR (2) lying down – to a standing, upright position.

Many people will experience some degree of orthostatic hypotension while doing a water-only fast. (R)

Severity of orthostatic hypotension during a water fast may be contingent upon electrolyte status, water intake, and duration of the fast.

Orthostatic hypotension associated with water fasting may be described as transient bouts of dizziness, lightheadedness, blurred vision, and/or faint feelings – each time you stand up.

If you experience orthostatic hypotension and aren’t careful, you may end up fainting – which could result in injury (and in extreme cases, death).

Management: It may be possible to manage fasting-induced orthostatic hypotension via a combination of: getting up slowly and mindfully flexing muscle groups (e.g. the abdomen) prior to standing upright from seated or prone positions.

Research suggests that keeping the head elevated whenever sitting or lying down and wearing compression clothing may help prevent or reduce the severity of orthostatic hypotension.

Supplementing with electrolytes might also help manage orthostatic hypotension (as electrolyte deficits/imbalances can directly cause orthostatic hypotension). (R1, R2, R3)

Low blood pressure

Water fasting frequently causes blood pressure to decrease.

In some cases, the blood pressure reduction is significant enough to be classified as “hypotension” (less than 90/60 mm Hg).

Low blood pressure may be accompanied by myriad symptoms (regardless of one’s posture), including: dizziness, lightheadedness, blurred vision, fatigue, and nausea.

Substantial reductions in blood pressure while water fasting can also increase risk of fainting and the number/severity of orthostatic hypotension episodes.

If you constantly feel dizzy, lightheaded, and/or you end up fainting during a fast – realize that these reactions are probably due to significant reductions in blood pressure.

Management: It is recommended to regularly monitor your blood pressure while water fasting to ensure that numbers don’t drop too low.

Furthermore, supplementation with electrolytes (e.g. sodium) may help prevent hypotension or reduce the magnitude of hypotension during a water fast.


Another extremely common side effect of water fasting is dehydration – a significant loss of bodily fluid to the extent that normative physiologic functions become impaired.

Some individuals may become dehydrated during a fast due to a combination of:

  • Adipsia or hypodipsia (and a corresponding failure to drink enough water)
  • Water weight loss
  • Excessive sweating (i.e. hyperhidrosis)
  • Diarrhea
  • Electrolyte imbalances

Common signs of dehydration include: dry mouth, muscle cramps, heart palpitations, weakness, emetic reactions (e.g. nausea and vomiting), cessation of sweating, dry eyes, dizziness, and fatigue.

Management: To prevent and/or manage dehydration throughout a water fast – be sure to consume sufficient water each day and correct all electrolyte imbalances via supplementation.

If you experience adipsia (lack of thirst) or hypodipsia (decreased thirst) – you may want to force yourself to consume a specific amount of water per day to ensure that these reactions won’t lead to dehydration.

If dehydration ever becomes severe, it is recommended to seek emergency medical care.

Lower back pain

Many people complain of lower back pain during water fasts – particularly during the first few days.

Some fasting practitioners suspect that the emergence of lower back pain during a water fast is attributable to changes in kidney function.

The changes in kidney function while fasting are hypothesized to “refer” pain to the lower back.

Other potential causes of lower back pain during a water fast include: physical inactivity (many people remain sedentary during water fasts and sit/lie in awkward positions); dehydration; and electrolyte imbalances.

In many cases, lower back pain diminishes in intensity and abates within 3 to 5 days once physiology adapts to a completely fasted, ketotic state.

Management: If you experience lower back pain during a water fast, it is recommended to ensure that you’re consuming adequate water and supplementing with electrolytes.

If the back pain becomes severe and/or unbearable – seek emergency medical care (as this pain could be a sign of fasting-related kidney dysfunction).

Tongue discoloration

After a few days of water fasting, many people notice tongue discoloration – “white tongue” is most common.

Misinformed persons generally attribute tongue discoloration (e.g. whitening) to “detox” (like many other side effects) – but this is generally inaccurate.

For most, the cause of tongue whitening while water fasting is an accumulation of keratin on the tongue’s surface.

The surface of your tongue contains 4 types of bumps known as “papillae” including: filiform, fungiform, circumvallate, and foliate.

Filiform papillae differ from other papillae (fungiform, circumvallate, foliate) in that filiform papillae: are small and conical-shaped; have zero taste buds; and have keratinized epithelial tips (i.e. surfaces covered in dead cells containing the protein keratin).

Anyone who stops consuming food for a while (such as during a water fast) will accumulate keratin on the tongue due to the fact that: (1) filiform papillae keep producing it; (2) there’s zero friction from food consumption to scrape it off; and (3) saliva production keeps keratin trapped between papillary crevices.

Predictably, keratin accumulation on the tongue yields a “white layer” on the tongue’s surface.

Though keratin accumulation may be the most common cause of a white tongue during water fasts – it is not the only possible cause.

A white tongue could also occur due to oral candidiasis (thrush) – resulting from a combination of a weakened immune system and changes in oral microflora.

Other explanations for tongue discoloration during a water fast include: dehydration, ketosis, fat loss, and detoxification.

Management: An effective way to manage tongue discoloration during water fasts is to brush the tongue vigorously (once or twice per day) and then rinse off the residue with warm water – this should help dislodge keratin from papillary crevices.

Maintaining healthy oral hygiene by brushing teeth and using mouthwash can also be helpful.

Staying hydrated might also prove useful for minimizing tongue discoloration during a fast.

If the tongue discoloration is caused by candidiasis (oral thrush) – it may be necessary to seek medical care to treat both the candidiasis and immune dysfunction.

(In some cases, ending the water fast will be the only way to reverse tongue discoloration).

Taste abnormalities

It is fairly common to experience taste abnormalities during water fasts.

Perhaps the most common taste abnormality while water fasting is a foul or disconcerting “metallic taste.”

Taste abnormalities during water fasts can have many potential causes, including:

  • Keratin accumulation
  • Candidiasis
  • Oral microbiome fluctuations
  • Dehydration
  • Ketosis
  • Fat loss & detoxification
  • Poor oral hygiene

In rare cases, some individuals will notice “sweet” and/or “fruity” tastes within the mouth while fasting – due to onset of ketoacidosis.

Should you ever experience a sweet or fruity taste in your mouth during a water fast – do not hesitate to seek emergency medical attention – as this could be ketoacidosis, a [potentially] life-threatening metabolic state.

Management: Regularly: (1) brushing your teeth with fluoridated toothpaste; (2) brushing your tongue (to eliminate keratin buildup), and (3) using ADA-certified mouthwash – can go a long way towards improving oral health during a fast.

Maintaining adequate hydration can also be helpful for combating taste abnormalities.

If you ever experience fruity or sweet tastes in your mouth while fasting – seek medical attention to rule out ketoacidosis.


After several days of water fasting, it is common to experience “bad breath” – medically referred to as halitosis.

Halitosis during a water fast may be accompanied by tongue discoloration and taste abnormalities throughout the mouth.

There are numerous potential causes of halitosis while fasting, including:

  • Keratin buildup
  • Candidiasis
  • Oral microbiome shifts
  • Ketosis
  • Dehydration
  • Fat loss & detoxification
  • Poor oral hygiene

Because many people abstain brushing the teeth/tongue and using mouthwash during water fasts – this could lead to the accumulation of debris (e.g. keratin) and other pathogenic bacteria within the mouth, resulting in halitosis.

Management: Assuming you don’t have candidiasis and aren’t malnourished, the most effective way to counteract fasting-induced halitosis is to stay hydrated, rinse your mouth out with water (regularly), brush your teeth/tongue, and use mouthwash (twice daily).

Brushing your teeth/tongue and using mouthwash will not break your fast.


While engaging in a water fast, most individuals will [predictably] experience hunger as a side effect – due to caloric abstinence.

Many have suggested that hunger tends to be most pronounced during the first 1 to 3 days of the water fast due to the fact that the body is still using glucose for energy.

When glucose levels plummet and the individual fasting becomes hypoglycemic (in a glucose-adapted state), this can send strong hunger signals to the brain wherein hunger sensations become overwhelming – making it difficult to continue the fast.

Once the body metabolically transitions from a glycolytic state (using glucose for energy) to a ketotic state (using ketones for energy), many individuals report that hunger [subjectively] decreases.

Despite the fact that hunger can decrease upon entering ketosis, this does not mean that hunger completely abates – as it is still possible to feel hungry while in a state of fasting-induced ketosis.

It is also thought that insufficient water intake and/or dehydration, along with electrolyte imbalances could exacerbate preexisting hunger.

In most cases, individuals with higher levels of body fat should experience less hunger throughout a fast than persons with lower levels of body fat – due to the fact that the former (fatter person) has more energy (i.e. fat) reserves than the latter.

After a prolonged period of fasting and/or once a low body fat has been achieved, it is hypothesized that a state of “true hunger” is reached wherein extremely intense hunger surfaces as a signal that the fast needs to end (or death will ensue).

Understand that not everyone has the willpower or motivation to cope with the hunger that surfaces during a water fast.

Management:  Hunger is hypothesized to be most intense during the first 1 to 3 days of water fasting due to the onset of hypoglycemia in a glycolytic state.

Once the body fully transitions to a ketotic state (relying fully upon ketones for energy) – hunger tends to [subjectively] decrease for many.

For this reason, many speculate that it might be possible to decrease acute hunger (in the first few days of water fasting) by entering ketosis (via a ketogenic diet) prior to the fast.

Entering ketosis via diet prior to water fasting should ensure that the body won’t experience hunger attributable to hypoglycemia in a glucose-adapted state.

Some individuals have reported that maintaining adequate hydration and electrolyte supplementation may help attenuate hunger sensations during water fasts.

Keeping yourself busy with regular activities (e.g. socializing, reading, working, watching TV, etc.) may also be an effective way to distract yourself from hunger or food cravings.

Assuming you’ve reached a low BMI or body fat percentage via water fasting – hunger sensations are a sign that you should break your fast.

Skin rash

Fasting practitioners report regularly observing skin rashes in clients/patients during water fasts.

Though there are many hypotheses as to why skin rashes occur during water fasts, no formal explanations have been offered.

Skin rashes can be triggered by many things during a fast, including:

  • Shifts in skin flora (cutaneous microbiome)
  • Stress hormones
  • Immune function
  • Resurfacing of infections
  • Ketosis
  • Dehydration
  • Electrolyte & nutrient deficits
  • Fat loss & detoxification
  • Poor hygiene

During a water fast, it is thought that there may be substantial changes in skin flora composition – such as to provoke a skin rash.

Entering ketosis can also cause skin rashes in select individuals – hence popularization of the term “keto rash.”

Moreover, because the body is often stressed during water fasts (due to lack of food), concentrations of stress hormones (e.g. cortisol) significantly increase.

High concentrations of stress hormones can directly trigger skin reactions and can impair immune function such as to enable resurfacing of latent infections (e.g. herpes viruses) – and thus, rashes of infectious etiologies.

A combination of dehydration, electrolyte deficits, and/or nutrient deficiencies could also trigger skin rashes while water fasting.

Because water fasting has been shown to remodel the immune system, it’s possible that during this remodeling, one might become increasingly sensitive to environmental allergens – wherein a rash occurs.

(Anyone water fasting with preexisting autoimmune conditions should be vigilant of changes in skin appearance to ensure that a rash isn’t due to an autoimmune flare-up).

It is also possible that a combination of rapid fat loss and corresponding detoxification could provoke skin rashes while fasting.

Finally, it is important to consider that select individuals may neglect hygiene while fasting such that the skin accumulates sebum, keratin, sweat, dirt, and debris – causing conditions like dermatitis neglecta – with noticeable skin rashes.

Management: To minimize likelihood of skin rashes during water fasts, it is recommended to keep your stress level low (as stress hormones are already high).

Getting plenty of sleep, engaging in paced breathing, and/or meditating should help decrease stress and attenuate any skin reactions resulting from high stress.

Maintaining adequate hydration and supplementing with electrolytes might also reduce the odds of a skin reaction while fasting.

Regularly bathing and washing the skin should minimize likelihood of skin reactions from the buildup of sebum, keratin, sweat, dirt, and debris.

If you want an accurate diagnosis of your skin rash while water fasting, it is recommended to consult a dermatologist.


Headaches are one of the most common side effects of water fasting.

According to research by Torelli et al. (2009), a “fasting headache” has the same clinical features as tension-type headache and probability of onset increases directly with duration of fasting. (R)

There are many possible causes of headaches while water fasting, including:

  • Dehydration
  • Electrolyte imbalances
  • Stress hormones
  • Hypoglycemia
  • Caffeine withdrawal
  • Ketosis (R)
  • Shift in brain activity

Anyone who becomes dehydrated or exhibits electrolyte imbalances during a water fast might develop a headache.

Additionally, it is understood that fasting can trigger acute increases in concentrations of stress hormones (e.g. cortisol).

Any significant spikes in stress hormones have potential to induce tension-type headaches throughout a fast.

We also know that hypoglycemia (low blood sugar) can cause headaches in select individuals – particularly in the first few days of fasting when the body is attempting to metabolically shift from glucose-adapted to ketosis.

Furthermore, because: (1) regular caffeine drinkers are forced to withdraw from caffeine while water fasting AND (2) headache is a common symptom of caffeine withdrawal – it’s logical to suspect that caffeine withdrawal causes headache during the water fast (for many people).

Research also suggests that headache might be a side effect of ketosis – such that once your body shifts to a ketotic state, mild headaches could occur.

Lastly, because fasting: (1) likely modulates brain activity to a significant extent (including neurotransmission, blood flow, regional activity, etc.) AND (2) shifts in brain activity can cause headaches – some individuals might experience headaches from activation changes within the brain.

Management: To minimize likelihood of headaches while water fasting, it is recommended to stay hydrated and supplement with electrolytes.

Engaging in regular stress reduction and getting plenty of sleep might help prevent headaches attributable to elevated stress hormone production while water fasting.

If your headaches become severe or unbearable during a fast, it is recommended to seek medical care and break the fast.


Low blood sugar, medically referred to as hypoglycemia, is a common occurrence while water fasting.

Assuming a glycolytic state prior to the water fast, the body will first metabolize readily-available glucose stores for energy in the first 1-2 days of the fast.

Because zero calories are consumed to provide additional glucose to fuel physiologic processes in a glucose-adapted state, glucose concentrations drop extremely low.

During this time, the body enters a hypoglycemic state in which a variety of symptoms might occur, including: shakiness, dizziness, sweating, hunger, irritability, anxiety, headache, and irregular heart rate.

Once the body fully shifts from a glycolytic state to a ketotic state, symptoms of hypoglycemia tend to diminish because ketones are efficiently used to fuel physiologic processes.

Management: There’s really no way to avoid hypoglycemia during a water fast if you initiate the fast from a glycolytic state.

However, if you enter ketosis prior to the water fast (via following a ketogenic diet), then you shouldn’t experience hypoglycemia-related symptoms during the water fast.

Fatigue or lethargy

Don’t be surprised if you experience extreme fatigue or lethargy while water fasting.

There are many possible reasons that someone might experience fatigue during a fast, including:

  • Hypoglycemia
  • Ketosis
  • Hormone fluctuations
  • Electrolyte imbalances
  • High stress & poor sleep

Hypoglycemia (low blood sugar) may be culpable for your fatigue during the first few days of water fasting.

Many individuals find that fatigue ceases upon completion of the metabolism shift from a glycolytic to a ketotic state.

Nevertheless, some individuals may experience fatigue while fasting directly due to ketosis – possibly coupled with lack of energy intake.

Furthermore, fluctuations in concentrations of various hormones including: thyroid hormone (T3, T4, etc.); sex hormones; and stress hormones – may all contribute to the manifestation of low energy while fasting.

Deficiencies or imbalances in electrolytes could also induce fatigue while fasted.

A combination of high stress (poor stress management) and poor sleep – might also exacerbate fatigue.

Lastly, anyone who has reached a low body fat percentage while fasting may experience extreme fatigue as a biological signal to reinstate feeding.

Management: It may be possible to prevent hypoglycemia-induced fatigue during a water fast by becoming ketotic (via a ketogenic diet) before fasting.

Entering the fast in ketosis should help prevent extreme symptoms attributable to hypoglycemia during the fast.  Supplementing with electrolytes to reverse imbalances might also improve energy levels in a fasted state.

Decreasing stress and proper sleep hygiene should also counteract fatigue.

Muscle cramps

Water fasting can cause muscle cramps as a side effect for select individuals.

The most likely causes of muscle cramps while water fasting include:

  • Dehydration or insufficient water intake
  • Electrolyte imbalances or deficits
  • Stress hormones

In a fasted state, the body efficiently excretes excess water such that it’s easy to become dehydrated.

Furthermore, some individuals may experience adipsia or hypodipsia such that thirst is lacking or diminished – possibly resulting in insufficient water intake.

Subclinical or clinical dehydration during a water fast can trigger muscle cramps.

It is also common for individuals to develop muscle cramps as a result of electrolyte imbalances or deficiencies while water fasting.

Additionally, in a fasted state, stress hormones (e.g. cortisol, CRH, catecholamines, etc.) may become elevated which can: increase muscle tension and modulate electrolytes – to cause muscle cramps.

In rare cases, water fasting might provoke kidney dysfunction in susceptible individuals wherein muscle cramping could result.

Management: If you’re developing muscle cramps while water fasting, it is recommended to ensure that: (1) your water intake is adequate and (2) your electrolyte levels are normal and balanced.

Moreover, it is recommended to utilize stress reduction tactics (e.g. deep breathing, meditation, etc.) and maintain good sleep hygiene to ensure that you get sufficient quality sleep.

If you have reason to believe that the cramps might be related to kidney dysfunction – seek immediate medical attention.


Most people who engage in water fasting will experience insomnia as a side effect – especially during the first few nights of the fast.

The insomnia might occur: (1) when you’re trying to fall asleep (sleep-onset insomnia) and/or (2) after you’ve been asleep for a while wherein you wake up and are unable to fall back asleep (sleep-maintenance insomnia).

During a water fast, concentrations of stress hormones increase because the body: (1) no longer receives energy at regular intervals via exogenous calories and (2) must transition from a glycolytic state to a ketotic state in which endogenous energy stores (e.g. body fat, muscle, etc.) are utilized for fuel.

Because stress hormone (e.g. cortisol, CRH, catecholamines, etc.) production can increase during a water fast (as a physiologic queue to secure food as soon as possible) – it frequently becomes difficult to fall asleep or maintain sleep throughout the night.

For some individuals, insomnia may occur early in the water fast (e.g. the first few nights) and completely abate once the body has completed its metabolic transition from glucose-adapted to ketosis.

Moreover, dehydration and electrolyte imbalances could also directly cause and/or exacerbate preexisting insomnia by interfering with the production of sleep-inducing neurotransmitters like melatonin.

Management: It may be impossible to fully mitigate insomnia during water fasts.

To reduce frequency and/or severity of insomnia during water fasts, it is recommended to maintain good sleep hygiene by avoiding: electronic devices, high stress activities, loud sounds, and bright light (especially blue light wavelengths) 1 to 3 hours before bed.

Also, ensure that your room temperature is set at a threshold conducive to sleep.

Managing stress, optimizing electrolytes (via supplementation), and maintaining adequate hydration should also help decrease frequency and/or severity of insomnia during a water fast.

Sleep disturbances

In addition to insomnia, a variety of additional sleep disturbances can emerge while water fasting.

During water fasts, people have reported:

  • Atypical/weird dreams (e.g. dreams about food)
  • Broken sleep (frequent awakenings)
  • Hypnagogic & hypnopompic hallucinations
  • Insufficient sleep (inability to sleep for adequate duration)
  • Light sleep (inability to reach deep stages of sleep)
  • Nightmares (highly unpleasant dreams)
  • Vivid dreams (extremely detailed dreams)

In many cases, these sleep disturbances are caused by a host of physiologic changes that occur during a fast – including alterations in brain activation.

It is understood that water fasting can significantly increase concentrations of cortisol and other stress hormones, particularly in the early days of the fast.

Heightened production of stress hormones could account for many of the aforementioned sleep disturbances associated with water fasting.

Hypoglycemia, ketosis, electrolyte imbalances, and dehydration – could also induce and/or exacerbate sleep disturbances while water fasting.

Management: Like insomnia, it may be impossible to completely mitigate sleep disturbances that emerge while water fasting.

Various sleep abnormalities should be expected while fasting as a result of heightened physiological stress associated with: (1) an absence of exogenous calorie intake; (2) the transition from glycolysis/gluconeogenesis to ketosis; and (3) rapid fat loss.

That said, good sleep hygiene might help to decrease the frequency and/or severity of sleep disturbances while fasting.

Adherence to a strict sleep/wake schedule, maintenance of an environment conducive to sleep (cool temperature, no light, no sound, etc.), and avoidance of activities and/or stimuli that interfere with circadian rhythms and/or sleep – might prove beneficial.

Engaging in daily stress reduction, staying hydrated, and optimizing electrolytes (via supplementation) might also decrease the frequency and/or severity of sleep disturbances while fasting.

Urine discoloration

Discoloration of urine while water fasting can occur – but should be addressed as soon as possible.

Extremely dark yellow urine, orange urine, brown urine, and reddish urine have been reported online by individuals engaging in water fasts.

In most cases, darkening of the urine is a sign that the body is dehydrated.

When dehydrated, the body attempts to conserve water stores and/or lacks adequate water to excrete within urine (in comparison to when the body is adequately hydrated).

As a result, urinary excretion in a dehydrated state consists of highly concentrated urea and toxins (with limited dilution from water) – causing urine to appear dark or discolored.

Modest electrolyte deficiencies might also induce slight changes in urine color for some individuals during water fasts.

Additionally, in a fasted state, the body rapidly burns through adipose tissue for fuel – and this adipose tissue often contains toxins (e.g. persistent organic pollutants, drug metabolites, etc.).

These toxins are then released into the bloodstream and excreted within urine at higher concentrations than usual – possibly contributing to urine discoloration.

Nonetheless, if urine appears dark or discolored despite drinking sufficient water and supplementing with electrolytes – it may be smart to consult a medical doctor to rule out serious adverse reactions.

In rare cases, water fasting might trigger kidney dysfunction and/or contribute to the onset of rhabdomyolysis (especially among persons who exercise intensely while in a fasted state).

Management: Fluctuations in urine color can be normal for some individuals during a water fast.

If the urine becomes dark, it is recommended to consume additional water and/or electrolytes – as this could be a sign of dehydration.

Individuals who experience dark urine and/or decreased urinary output despite drinking plenty of water and supplementing with electrolytes – should seek medical care to evaluate kidney function.

Heart palpitations

Palpitations are defined as rapid, irregular, and/or abnormal beats of the heart – and frequently occur as a side effect of water fasting.

Many people describe palpitations as unexpected heart flutters and/or loud pulses.  To some, it may feel as if the heart is skipping beats – which can cause anxiety.

According to medical data, most palpitations (~70%) are of cardiac or psychiatric origin – and the remaining cases classified as miscellaneous or idiopathic origin. (R)

During a water fast, heart palpitations can occur for a variety of reasons, including:

  • Stress hormones & sympathetic activation
  • Hypoglycemia
  • Low electrolyte levels
  • Poor sleep
  • Dehydration
  • Cardiac dysfunction

It’s logical to speculate that heart palpitations are more likely to occur in the early days of a water fast due to the fact that activation of the sympathetic nervous system temporarily upregulates as a result of physiology adapting to a fasted state.

Acute upregulation of the sympathetic nervous system increases the production of stress hormones like cortisol, CRH, catecholamines, etc. – triggering palpitations.

A combination of hypoglycemia (especially while transitioning from a glycolytic to a ketotic state); low levels of electrolytes (e.g. magnesium); insufficient sleep; and dehydration – could also cause OR exacerbate preexisting palpitations during a fast.

Furthermore, persons at low body weights or histories of cardiac events/conditions prior to a fast may experience palpitations as a result of fasting-induced cardiac atrophy or dysfunction.

Management: In most healthy individuals (normal body weights without histories of cardiac events), some palpitations may occur during the initial few days of the water fast – but should subside as physiology adjusts to a fasted, ketotic state.

Once adapted to a fasted, ketotic state – activation of the parasympathetic nervous system usually increases to attenuate and override the sympathetic nervous system (which may have triggered a “fight-or-flight” response in the early days of the fast).

Lastly, realize that a combination of: stress management; adequate/quality sleep; sufficient hydration; and electrolyte supplementation – may be effective prophylactics and attenuators of palpitations.

If you have a history of cardiac events or conditions, do not attempt a water fast without the authorization and/or supervision of a cardiologist.

Similarly, if you’re already at a low body weight or have fasted for an extended duration to reach a low body weight – it is recommended to properly break your fast with medical supervision – as heart palpitations may be a sign of fasting-induced cardiac dysfunction.

Gastrointestinal reactions

Fasting may also trigger gastrointestinal reactions like diarrhea, constipation, bloating, or stomach aches, and/or stomach growling/gurgling noises.

Causes of gastrointestinal reactions during a water fast might include:

  • Pre-fast food intake
  • Acute stress
  • Electrolyte imbalances
  • Dehydration
  • Gut microbiota shifts

First off, it’s important to understand that: (1) the quantity of food you consumed prior to your fast AND (2) the specific foods you consumed prior to your fast – could account for gastrointestinal reactions that you notice during the first 1 to 3 days of your fast.

For example, if you binge ate 15,000 calories of a large pizza, breadsticks, ice cream, cake, cookies, and cereal as the final meal prior to your fast – you may experience gastrointestinal reactions solely due to digestion and excretion of this calorie bolus.

The specific composition (fiber, sugar, carbs, fat, protein, etc.) of the food(s) you consumed prior to your fast may also impact your digestion during the first few days of a fast.

Assuming your gastrointestinal reactions while fasting aren’t attributable to the specific foods or quantity of calories that you consumed prior to your fast – then things like: hunger, stress, electrolyte imbalances, dehydration, and shifts in the gut microbiome may explain gastrointestinal reactions.

Management: Assuming the gastrointestinal reactions you experience while fasting occur during the first few days of your fast – it’s very likely that these reactions will eventually subside.

Because stress can aggravate the gastrointestinal tract and alter composition of the gut microbiome, it is recommended to (1) engage daily relaxation exercises and (2) get adequate/quality sleep – to minimize the stress response.

Maintaining adequate hydration and supplementing with electrolytes might also help counteract gastrointestinal complications that emerge during a water fast.

If you have a history of gastrointestinal disorders/conditions, never attempt a water fast without authorization and/or supervision of a gastroenterologist.

If the gastrointestinal complications that you experience while fasting are severe – do not hesitate to seek medical attention.

Brain fog or cognitive impairment

While water fasting, most people will “brain fog” (i.e. unclear thinking) and cognitive impairment.

Brain fog and cognitive impairment during a water fast can occur as a result of:

  • Hypoglycemia
  • Zero energy intake
  • Electrolyte deficits
  • Dehydration (R)
  • Autonomic shifts
  • Brain function

In the early stages of a water fast, brain fog and cognitive impairment are likely attributable to hypoglycemia (low blood sugar).

Research suggests that – relative to euglycemia (adequate blood sugar) – hypoglycemia significantly impairs nearly every aspect of executive function in adults. (R)

A study in amateur weightlifters showed that a 48-hour fast significantly decreased frontal brain activity and impaired cognitive function. (R)

Once physiology fully shifts from a glycolytic state (reliant upon glucose for energy) to a ketotic state (reliant upon ketones for energy), any initial brain fog and cognitive impairment attributable to hypoglycemia may decline due to the fact that the brain adapts to receiving ketones for maintenance of its functions.

Some individuals may experience brain fog or cognitive impairment while fasting due to the fact that zero calories are consumed, resulting in food cravings and feelings of hunger.

If you’re constantly feeling hungry and craving foods but need to maintain mental discipline to abstain from eating – this could tax your willpower to the extent that it impairs cognitive function.

Other common causes for brain fog and cognitive impairment while fasting are electrolyte deficits, dehydration, shifts in activation of the autonomic nervous system (e.g. parasympathetic dominance/sympathetic suppression), and altered brain function (regional activation, neurotransmission, etc.) – as a result of the fasted state.

Management: If you experience brain fog or cognitive impairment while fasting, realize that these side effects are probably attributable to a combination of: (1) hunger/food cravings – which are distracting and tax willpower; (2) hypoglycemia – prior to ketosis; (3) electrolyte imbalances; and/or (4) dehydration.

After the body shifts from glycolysis/gluconeogenesis to ketosis, many people notice that hunger/cravings diminish and clarity of thought improves due to the fact that physiology becomes keto-adapted wherein ketones are continuously supplied to the brain.

Maintaining adequate hydration and supplementing with electrolytes may reduce brain fog and cognitive impairment while water fasting.

Additionally, managing stress and getting enough quality sleep to preserve healthy autonomic nervous system activation may also prove helpful to maintain cognitive function and/or clarity of thought during a fast.

Nevertheless, it is recommended to avoid planning a fast during periods in which peak cognition is necessary (e.g. academic exams, cognitively-demanding work, etc.).


Fainting, also referred to as syncope, is considered a brief loss of consciousness – frequently induced by hypotension.

Because hypotension (low blood pressure) and orthostatic hypotension (low blood pressure upon standing from sitting/lying positions) commonly occur while water fasting, it’s logical to suspect that these side effects trigger fainting episodes in a subset of persons.

A combination of electrolyte deficiencies (e.g. hyponatremia) and dehydration can also increase one’s risk of fainting during a water fast.

Understand that fainting prophylaxis should be taken seriously – as just one unlucky episode could result in serious injury (e.g. concussion) or death.

Management: To prevent fainting episodes while water fasting, you may want to monitor your blood pressure to ensure that it doesn’t dip excessively low.

If you experience hypotension (low blood pressure) or orthostatic hypotension (low blood pressure accompanied by lightheadedness and/or dizziness each time you stand up) – consider supplementing with electrolytes like sodium to increase your blood pressure.

Additionally, things like: keeping your head elevated when seated, wearing compression clothing, and/or contracting various muscles (e.g. abdominal) prior to standing up may help prevent fainting as a result of orthostatic hypotension.

Since fainting can occur from dehydration, ensure that you’re consuming adequate water (along with electrolytes) to prevent this from happening during the fast.

Eustachian tube dysfunction (ETD)

Eustachian tube dysfunction is a side effect of water fasts that’s seldom discussed.

The most common type of Eustachian tube dysfunction that occurs during fasting (or rapid fat loss) is Patulous Eustachian tube (PET).

Patulous Eustachian tube is a condition in which the Eustachian tube of an ear remains intermittently open – instead of staying closed.

Fasting causes Patulous Eustachian tube by inducing fat loss directly around the Eustachian tube.

Since the fatty supports around Eustachian tubes are necessary for Eustachian tube closure, significant loss of these fatty supports can cause the Eustachian tube to open intermittently – resulting in Patulous Eustachian tube.

Symptoms of Patulous Eustachian tube include: hearing loss, tinnitus (ringing in ears), clogged ears, suction-like sensations within the ears, autophony (echoing within the head while speaking), and/or crackling or popping sounds within the ears.

Though Patulous Eustachian tube can be extremely distressing, it is usually reversible by regaining body fat.

Although rapid fat loss is the most likely cause of Eustachian tube dysfunction while fasting, other things like: elevated stress, dehydration, and electrolyte imbalances – may also play roles.

(And no, changes in ear function and/or hearing while fasting are usually not caused by “toxins being released”).

Management:  For persons with Eustachian tube dysfunction caused by stress, dehydration, and/or electrolyte imbalances – things like stress reduction, adequate hydration, and electrolyte supplementation should help normalize Eustachian tube function.

That said, because body fat loss is the most common cause of Eustachian tube dysfunction while fasting – it is usually recommended to stop fasting (properly break the fast) if this side effect occurs.

If you continue fasting with Eustachian tube dysfunction, additional fat loss may increase episodes of dysfunction and/or the severities of your symptoms.

Regaining body fat and increasing dietary fat intake should help reverse most cases of fasting-induced Eustachian tube dysfunction.

In the event that fat gain doesn’t normalize your Eustachian tube dysfunction – consult an otolaryngologist.

Psychiatric symptoms

Though many people report significant improvements in mental wellbeing or psychiatric disorders (e.g. anxiety, depression, etc.) during water fasts – others might report the opposite.

Examples of common psychiatric symptoms that might occur while water fasting include:

  • Anger
  • Anxiety
  • Depression
  • Irritability
  • Mood swings

There are myriad potential causes for psychiatric symptoms during water fasts, including:

  • Hypoglycemia
  • Hormone levels
  • Stress & poor sleep
  • Dehydration
  • Electrolyte deficiencies
  • Ketosis

Hypoglycemia, which usually becomes most pronounced within the first 48 hours of fasting, can trigger a variety of psychiatric symptoms due to the fact that physiology is stressed from lack of calories in a glycolytic state.

As a result, stress hormone (e.g. cortisol) production acutely increases which interferes with sleep and can cause mood to tank.

Once physiology shifts from a glycolytic state to a ketotic state (using ketones for energy), stress hormone production decreases – possibly yielding mood stabilization.

The emergence of unpleasant psychiatric symptoms during a fast may also be associated with upregulation of orexin/hypocretin (a neurotransmitter involved in the promotion of wakefulness).

Dehydration and electrolyte imbalances may also be culpable for the induction of unwanted psychiatric symptoms while water fasting. (R)

Moreover, some persons may experience psychiatric symptoms directly as a result of ketosis – as not everyone’s brain activity/neurochemistry is favorably affected by ketones.

Management: If you want to engage in a water fast, understand that mood swings and other unwanted psychiatric symptoms may occur as side effects – even among persons without histories of psychiatric abnormalities.

If you have a history of psychiatric disorders or episodes – it is recommended to receive authorization from a psychiatrist prior to engaging in the fast to reduce odds of adverse psychiatric reactions.

For some individuals: adequate hydration, electrolyte supplementation, stress management, and/or good sleep hygiene may help prevent or mitigate mild psychiatric symptoms that emerge while fasting.

If psychiatric symptoms become severe and/or unmanageable during your water fast – it is recommended to discontinue the fast (break it safely) and consult a medical professional.


Nausea, or a feeling as though you’re about to vomit, is another common side effect of water fasting.

Though it usually doesn’t become severe enough to provoke vomiting, nausea can be disconcerting and uncomfortable for many.

Various causes of nausea while water fasting can include:

  • Hypoglycemia
  • Hunger
  • Stress hormones
  • Dehydration
  • Electrolyte deficiencies
  • Ketoacidosis
  • Organ dysfunction

In the early stages of a fast, some individuals may report nausea as a result of hypoglycemia or hunger sensations – as hunger itself can cause some individuals to experience transient nausea.

Furthermore, a physiologic stress response (generated via the sympathetic nervous system) may occur for some individuals in the early stages of fasting – which could provoke nausea.

Dehydration and electrolyte deficiencies while fasting may also cause nausea in a fasted state.

In rare cases, nausea accompanied by other symptoms may be a sign of a serious adverse reaction to water fasting like ketoacidosis or organ dysfunction.

Management: Though nausea is both a common and transient side effect of water fasting – it may be unpleasant.

It may be possible to prevent some nausea during a water fast by staying hydrated, supplementing with electrolytes, avoiding/managing stress, getting enough sleep, and/or following a ketogenic diet prior to the fast (to avoid the stress and/or hunger response associated with transitioning from glycolysis/gluconeogenesis to ketosis).

If nausea becomes severe while fasting, seek medical attention to rule out serious adverse reactions like ketoacidosis and organ dysfunction.

Muscle loss

Most people who engage in water fasting aim to lose body fat – while simultaneously improving biomarkers.

However, research suggests that water fasting can cause muscle loss as a side effect.

A paper by Kerndt et al. (1982) suggests that, early in fasting, a high rate of gluconeogenesis is observed with amino acids as primary substrates. (R)

In other words, the body is burning muscle tissue as its primary fuel source via gluconeogenesis following glycogen depletion.

Eventually, ketone levels will rise and replace glucose as the primary fuel source in the central nervous system such that gluconeogenesis and protein catabolism should decrease.

A study by Jackson et al. (2006) reported that a man weighing 211.64 lb. (pre-fast) lost ~54 lbs. after a 44-day fast to reach a new weight of 157.63 lbs (post-fast).

Of the ~54 lb. weight loss: 42.4% was muscle tissue (22.93 lbs.); 31.4% was non-muscular fat-free tissue (16.98 lbs.); and 26.1% was body fat (14.10 lbs.).

This supports the idea that prolonged water fasts tend to reduce muscle tissue and non-fat muscle tissue to a greater extent than body fat. (R)

Management: Some muscle loss will occur during a water fast.  The muscle loss may be more significant and/or apparent in persons who are glucose-adapted prior to the fast.

It may be possible to prevent and/or reduce the magnitude of muscle loss while water fasting by entering ketosis (via a ketogenic diet) prior to the water fast.

If you’re already in ketosis before the water fast, your body will be less likely to catabolize muscle into glucose.

Note: These are some of the more common reactions to water fasting – and may not include every single “water fasting” side effect.  If you know of additional common side effects and/or want to share the side effects that you experienced – leave a comment.

Water fasting adverse reactions

Water fasting can cause severe adverse reactions in select individuals.

Understand that while most of these severe adverse reactions are rare (especially in healthy adults) – they can occur in anyone.

Individuals: (1) with preexisting medical conditions; (2) a history of medical episodes/events; (3) with low body weights; (4) using medications/supplements; (5) at risk for adverse reactions to fasting – should NOT water fast without prior authorization from a medical doctor.

Blood glucose abnormalities

Most evidence suggests that persons with diabetes can engage in water fasts if blood glucose levels are monitored and managed.

For example, research by Reiter et al. (2007) discovered that most patients with type 1 diabetes (37 of 56) can successfully complete a prolonged fast (greater than 25 hours) with no serious side effects – insofar as (1) blood-glucose is monitored and (2) insulin doses are adjusted. (R)

However, blood glucose abnormalities like: hypoglycemia (low blood sugar) and hyperglycemia (high blood sugar) can occur in patients with diabetes – particularly those with type 1 diabetes or advanced stage type 2 diabetes.

That said, failure to monitor and manage blood glucose during a water fast among patients with type 1 diabetes or advanced stage type 2 diabetes could yield severe hyperglycemia or hypoglycemia.

Symptoms of hyperglycemia include: thirst, headaches, blurred vision, fatigue, frequent urination, weight loss, confusion, coma, and fruity-smelling breath.

Symptoms of hypoglycemia include: shakiness, dizziness, sweating, hunger, irritability, anxiety, headache, and unconsciousness.

Because severe diabetic hyperglycemia or hypoglycemia can cause death – it is important to be cognizant of blood sugar levels and insulin doses while fasting.

Seek emergency medical attention if blood sugar abnormalities are suspected or detected.

Cardiac dysfunction

For individuals with preexisting cardiovascular conditions or histories of cardiovascular events – water fasting has potential to induce cardiac dysfunction.

Prolonged water fasts, particularly in persons of low body weight might cause cardiac dysfunction (e.g. ventricular arrhythmias) as a result of cardiac muscle atrophy.

In some cases, sudden weight loss can damage blood vessels which could: provoke fluctuations in blood pressure and heart rate/rhythm and increase risk of heart failure.

For example, a paper by Kerndt et al. (1982) highlighted several deaths from congestive heart failure and ventricular dysrhythmias among individuals while fasting or post-fast. (R)

That said, the deaths generally occurred in persons with preexisting heart problems OR in persons following liquid protein diets.

Most patients who died during or post-fast exhibited either QT prolongation, ST segment abnormalities, and/or T wave abnormalities – causing ventricular tachycardia and ventricular fibrillation.

Consult a cardiologist if you suspect that water fasting deleteriously affected your heart function.

Eating disorders

Some evidence suggests that water fasting may trigger the development or relapse of an eating disorder in susceptible individuals.

A study by Stice et al. (2010) examined the eating habits of 496 adolescent females over a 5-year span and discovered that fasting was a strong risk factor for binge eating disorder and bulimia nervosa. (R)

Although it’s certainly possible that the act of fasting (going without food) is nothing more than a consequence of eating disorders (rather than a cause), it’s also possible that water fasting provokes eating disorders in some cases.

For this reason, anyone with a history of eating disorders should refrain from fasting unless authorized by a psychiatrist who’s familiar with the patient’s eating disorder history.


Regular water fasting may increase your risk of developing gallstones – small crystalline masses formed in the gallbladder or bile ducts from bile pigments (e.g. bilirubin), cholesterol, and calcium salts.

Rapid weight loss (which occurs during prolonged water fasts) can also increase risk of gallstone formation in a subset of persons.

Why? Because when the body metabolizes fat, it causes the liver to secrete extra cholesterol into bile.

Symptoms of gallstones include: pain in the upper abdomen, back, or right shoulder that persist for hours; nausea and vomiting; fever; jaundice; and digestive complications (bloating, indigestion, heartburn, gas, stool discoloration).

A paper by Bloch et al. (1980) suggests that fasting for 10 to 20 hours may increase risk of gallstone formation – whereas prolonged fasts are thought to counteract this risk. (R)

If you suspect that regular fasting caused gallstones, do not hesitate to seek medical evaluation – and inform your provider about your fasts.


Though gout flares are uncommon during water fasts – they can occur in subsets of individuals.

While water fasting, kidney function can slightly decline and concentrations of uric acid can significantly increase.

It is suspected that uric acid concentrations significantly increase because keto acids compete with uric acids for renal tubular transport sites.

Since keto acids significantly rise while water fasting, the kidneys become unable to efficiently excrete uric acids – leading to hyperuricemia. (R)

One study reported uric acid levels of 9.0 mg/dl (normal is 3-4 mg/dl) in a non-obese individual on the final two days of a 45-day water fast. (R)

In a separate study of obese patients who fasted for up to 4 months, serum concentrations of uric acid increased progressively for the first 15 to 20 days – reaching 12-18 mg/dl. (R)

Among individuals with a history of gout, fasting-related elevations in uric acid may cause mild, moderate, or severe gout symptom flare-ups (e.g. gouty arthritis) during a fast.

If you have a history of gout, it is recommended to consult a rheumatologist: (1) to ensure that fasting is safe with your condition; and (2) for instruction regarding what to do if you experience gout during your fast.


Ketoacidosis is a life-threatening condition characterized by: (1) ketonemia; (2) hyperglycemia; and (3) metabolic acidosis – and can occur while water fasting.

It most often occurs in patients with type 1 diabetes or late-stage type 2 diabetes – but nondiabetic populations are sometimes affected.

Risk factors for ketoacidosis include: elevated blood sugar, elevated ketones, insufficient insulin production or administration, caloric restriction/fasting, dehydration, stress, alcohol/drug use, and infections.

Common symptoms of ketoacidosis include: excessive thirst, fruity-scented breath, confusion, weakness, fatigue, abdominal pain, frequent urination, nausea, and vomiting.

A paper by Burge et al. (1993) reported that a short-term (32-hour) fast can induce euglycemic ketoacidosis (ketoacidosis with normal blood sugar levels) in patients with type 1 diabetes during periods of insulin deficiency. (R)

A case report by Mostert and Bonavia (2016) documented starvation ketoacidosis in a healthy 24-year-old female as a result of fasting for surgery. (R)

A case report by Hudak et al. (2015) documented ketoacidosis in a 32-year-old female who was fasting during lactation due to abdominal pain. (R)

A case report by Joseph et al. (2009) documented starvation-induced euglycemic ketoacidosis in a 34-year-old male with type 1 diabetes.

If you suspect ketoacidosis while fasting (regardless of whether hyperglycemic or euglycemic), seek emergency medical attention.

Kidney dysfunction

Water fasting could cause kidney (renal) dysfunction in subsets of persons – particularly those with histories of kidney disease or impairment.

A study by Mojto et al. (2018) reported that water fasting for 11 days slightly impaired kidney function in 10 volunteers – resulting in decreased glomerular filtration rate and increased uric acid and creatinine. (R)

Although kidney function was slightly impaired in the 10 volunteers, the impairment was not clinically relevant – as measurements of kidney function remained within normal reference ranges.

A report by Kerndt et al. (1982) noted that fasting can cause urate nephrolithiasis (kidney stones comprised of uric acid) with subsequent renal insufficiency. (R)

Some speculate that the competition of ketone acids with uric acids for renal tubular transport sites can burden the kidneys – possibly inducing or exacerbating kidney dysfunction in susceptible individuals.

A study by Bakhit et al. (2017) reported worsening of renal function in patients with chronic kidney disease (CKD) while Ramadan fasting. (R)

Symptoms of renal dysfunction can include: infrequent urination or urine retention, shortness of breath, fatigue, confusion, nausea, weakness, chest pain, and/or edema (fluid retention).

If you suspect renal dysfunction during a fast, seek emergency medical care and a kidney panel from a nephrologist.

Liver dysfunction

In rare cases, water fasting may provoke or exacerbate liver (hepatic) dysfunction.

A study by Horiuchi et al. (1992) noted that a 40-hour fast increased alkaline phosphatase (ALP) by around 30% -suggesting that liver function can be altered by fasting. (R)

A case report by Elliott et al. (2016) – in which a 34-year-old male engaged in a 50-day self-imposed fast – stated that the patient’s plasma samples were suggestive of early impairment of liver function. (R)

Symptoms of liver dysfunction include: jaundice, fatigue, fever, appetite loss, nausea, vomiting, abdominal tenderness, dark urine, itchy skin, and pale stool color.

If you have a history of liver impairment, do not attempt a water fast without prior authorization from a hepatologist.

In the event that you suspect liver dysfunction during your fast – seek emergency medical care.

Psychiatric reactions

It is understood that psychiatric symptoms such as anger, anxiety, depression, and mood swings can occur during a fast.

In most cases, psychiatric symptoms are of mild intensity and transient (such that they eventually dissipate).

However, in select individuals, fasting may induce severe psychiatric symptoms such as:

  • Anger outbursts
  • Depressive episodes
  • Delusions
  • Hallucinations
  • Mania or hypomania
  • Panic attacks
  • Suicidal thoughts

Individuals with formally-diagnosed psychiatric disorders such as: bipolar disorder, major depressive disorder (MDD), panic disorder, psychosis, and schizophrenia – should only attempt a water fast if deemed acceptable by a psychiatrist.

Why? Because fasting can significantly modify brain activity, neurochemistry, and hormone concentrations – which could trigger symptom relapse or exacerbation.

Some researchers suspect that fasting may cause psychotic symptoms in certain individuals via acute hyperactivity of the dopaminergic system. (R)

What might trigger adverse psychiatric symptoms during a water fast?

  • Hypoglycemia
  • Stress & sleep disturbances
  • Dehydration
  • Electrolyte deficiencies
  • Ketosis

Adverse psychiatric symptoms that occur from water fasting are most likely to occur in persons: (1) with preexisting psychiatric disorders; (2) using or withdrawing from alcohol, drugs, medications, or supplements.

Nevertheless, serious adverse psychiatric symptoms can occur in anybody during a prolonged water fast – especially if the person is highly stressed, unable to sleep, dehydrated, and/or exhibits electrolyte or nutrient deficits. (R)

If you experience adverse psychiatric reactions from water fasting – seek emergency medical attention to minimize likelihood of deleterious outcomes.


According to Alan Goldhamer (chiropractor), operator of a fasting clinic that has monitored over 10,000 patients through water fasts – vomiting is a relatively common adverse reaction. (R)

Although vomiting may be a transient reaction attributable to physiology adjusting to a fasted state – it might also be recurrent and/or the sign of something serious.

Things that could cause frequent vomiting during a water fast include:

  • Gastric distress
  • Elevated stress
  • Severe nausea
  • Ketoacidosis
  • Organ dysfunction

If you find yourself vomiting while fasting, understand that vomiting can cause severe dehydration and electrolyte deficiencies – so be sure to increase hydration and supplement with electrolytes.

In the event that vomiting is uncontrollable and/or frequent – seek emergency medical care, as this could be a sign of life-threatening reactions like ketoacidosis or organ dysfunction.

Vitamin & mineral deficiencies

Anyone who water fasts: (1) for a prolonged period; (2) at a low body weight; and/or (3) with preexisting vitamin and mineral deficiencies – could end up permanently damaging their health.

A study by Jackson et al. (2006) involving a 44-day water fast by a non-obese male reported losses of: manganese (1.2%), copper (2.3%), molybdenum (4.4%), sodium (12.8%), selenium (17.3%), vitamin K (28.1%), and zinc (40.5%).

It is estimated that around 33% of individuals will develop mild hyponatremia (low sodium) within 6 to 24 days of fasting.

In the study by Jackson et al., biochemical evidence of deficiencies in thiamine (B1), riboflavin (B2), and vitamin K – were documented. (R)

Individuals fasting for a short duration, at high body weights (overweight/obese), with adequate vitamin stores – are less likely to develop serious vitamin and mineral deficiencies while fasting.

Nonetheless, vitamin and mineral deficiencies can occur in any person during a fast.

If you suspect you’re developing a vitamin or mineral deficiency during a fast, it is recommended to: (1) properly break the fast, (2) have a medical doctor check your vitamin/mineral levels, and (3) treat deficiencies.

Variables that influence water fasting side effects…

Why might some people experience marked differences in: specific side effects, rates of side effect onsets, side effect severities – relative to another person – during a water fast?  Individual variation.

Variables like: (1) the specific person fasting (age, body composition, genotype, medical status, etc.); (2) duration of the fast; and (3) whether supplements are administered during the fast – can all influence water fasting side effects.

1. The Specific Person


The age of a person entering the water fast may influence the side effects experienced.

It is hypothesized that pediatrics (children and adolescents) are at greatest risk of serious side effects while water fasting due to the fact that their brains and bodies are still developing.

Compared to healthy adults, elderly adults may also be at increased risk of serious side effects while water fasting due to age-related decline in organ function and muscle retention.

Research also suggests that elderly adults tend to live longer and perform better mentally if they are slightly overweight with extra muscle mass; fasting decreases body fat and catabolizes muscle.

Because critical aspects of brain development may persist until the mid-to-late 20s, it may be detrimental for young adults to engage in water fasts.

Nonetheless, it is thought that healthy adults are at lower risk of serious adverse reactions while water fasting relative to pediatric and elderly populations.

Body composition (pre-fast)

The body composition of an individual prior to entering the water fast (height, weight, body fat, lean mass, etc.) can influence onset rates and severities of side effects.

A 6-foot tall overweight/obese individual with adequate lean mass has greater energy stores (and probably vitamin/mineral stores) than a 6-foot tall normal weight or underweight individual.

For this reason, we would expect the overweight/obese individual to experience fewer side effects in a short-term fast relative to the normal weight individual.

Additionally, an overweight/obese individual is generally capable of a longer-term fast than a normal weight individual – and onsets of various side effects will be delayed relative to a normal weight counterpart.

An underweight individual who engages in a water fast could end up with permanent neurological damage – and should not be fasting in the first place.

Diet (pre-fast)

Diet before fasting can impact side effects that you experience while fasting.

For example, someone follows a ketogenic diet prior to his/her water fast (such that they are fully keto-adapted) should experience fewer acute side effects than persons following a diet with high or moderate carbohydrates.

Why? Because someone who transitions to a ketogenic diet before the water fast will be metabolically adapted to burning ketones instead of glucose.

Many side effects that occur early in a water fast are attributable to hypoglycemia and electrolyte losses while metabolically shifting from glycolysis/gluconeogenesis to ketosis.

Side effects like severe hunger, brain fog, heightened stress, and muscle loss should be: (1) less significant in those who are keto-adapted prior to the water fast and (2) more significant in those who follow high carbohydrate diets.

In addition to macronutrient composition of diet (and whether someone is keto-adapted), things like: (1) total caloric intake (prior to the fast) and (2) nutrient density of one’s diet – may impact fasting side effects.

Someone who consumes a calorie bolus (large quantity of calories) prior to fasting will require more time to reach a fully fasted state – than if the same individual consumed a small meal prior to his/her fast.

As a result, if the calorie bolus is consumed prior to fasting – onset of the fasted state and its side effects will be delayed relative to if a small quantity of calories is consumed prior to fasting.

Similarly, persons with nutrient-dense diets prior to entering fasts may be less likely to experience side effects attributable to nutrient deficiencies while fasting – relative to persons with diets devoid of nutrients.


A person’s genotype and epigenetic expression could influence the specific side effects that arise during a fast – as well as their onset rates and magnitudes.

Assuming we controlled for all other variables (e.g. body composition, height/weight, health, age, pre-fast diet, etc.), differences in water fasting side effects between two individuals would likely be due to genes and epigenetics.

Health status & medical conditions

Individuals with preexisting medical conditions or unhealthy lifestyles are at increased risk of side effects and adverse reactions during a water fast relative to healthy individuals.

Physiologic abnormalities attributable to preexisting medical conditions may exacerbate fasting-induced physiologic changes – thereby increasing (1) side effect intensity or prevalence (relative to healthy individuals) OR (2) symptoms of medical conditions.

Furthermore, fasting-induced physiologic changes may exacerbate physiologic abnormalities attributable to preexisting medical conditions – thereby increasing rates of medical condition relapse (among persons in remission) and/or the severities and numbers of symptoms associated with medical conditions.

If you have a medical condition or unhealthy habits, consider yourself at greater risk of side effects and adverse reactions while fasting – relative to healthy individuals.

Substance use

Anyone who (1) used substances regularly prior to fasting OR (2) uses substances while fasting – may experience differences in specific side effects and side effect intensity – relative to persons who remained sober before and throughout the fast.

Substances can be pharmaceutical medications, dietary supplements, illicit drugs, over-the-counter medications, legal drugs (e.g. alcohol, caffeine, cannabis, etc.).

For example, someone who regularly drinks coffee (e.g. 3 to 4 cups a day) might end up experiencing significant symptoms of caffeine withdrawal while entering the fast.

In this example, physiologic changes associated with caffeine withdrawal will give rise to “withdrawal symptoms” from caffeine (e.g. headache, fatigue, etc.) – and entering the fasted state will give rise to physiologic changes that produce side effects.

As a result of simultaneous (1) caffeine withdrawal symptoms and (2) physiologic adaptations to a fasted state – you may end up experiencing: a greater number of total “reactions” (i.e. side effects & withdrawal symptoms) and certain “reactions” may be more severe due to physiologic synergism.

It’s also important to acknowledge the possibility that withdrawing from certain substances could actually reduce the frequency of certain fasting-related side effects (due to physiologic counterbalance).

If you end up using substances while fasting, it is important to realize that certain substances (e.g. anxiolytics) might increase risk of certain fasting-related side effects like hypotension and fainting – while simultaneously decreasing risk of other side effects like anger and anxiety.

Other substances (e.g. psychostimulants) might decrease risk of certain fasting-related side effects like hypotension and fainting – while simultaneously increasing risk of other side effects like anxiety, anger, cardiac dysfunction, delusions, hallucinations, etc.

Additional substances like vitamins and minerals used strategically during a fast should help protect against risk of fasting-related side effects/adverse reactions – without downside (if properly dosed).

In summary: Using substances prior to or during a fast may increase risk of or the intensity of certain fasting-related side effects and/or adverse reactions – and might simultaneously decrease risk of others.  Certain substances (e.g. electrolytes) should only decrease risk of side effects if properly-dosed while fasting.


Biological sex (male vs. female) may influence the specific side effects and/or intensity of side effects that are experienced during a water fast.

It may be difficult to determine which specific side effects are more likely to occur in males relative to females during a water fast based upon the fact that men (on average) tend to have lower body fat and higher lean mass than women.

As was already mentioned, differences in body size and composition can account for differences in: (1) side effect onset; (2) specific side effects; and (3) side effect intensity.

A study by Browning et al. (2012) discovered that post-absorptive plasma fatty acids are higher in women than men – but increase more rapidly in men with the onset of early starvation.

Browning et al. also noted that sexual dimorphism was apparent in lean tissue triglyceride (TG) accumulation during a 48-hour fast – occurring in livers of men versus the muscles of women. (R)

Though the research by Browning et al. doesn’t prove that fasting side effect frequency and/or intensity tend to differ between males and females, it does confirm that there are sex-specific differences in certain biological adaptations to the fasted state.

Knowing this, it’s reasonable to speculate that sex-specific differences in biological adaptations to the fasted state could account for disparities between sexes in: side effect onset, specific side effects, and/or side effect intensity.

Vitamin & mineral status

The concentrations of vitamins and minerals within your body both prior to and during a fast – can influence rates of side effect onsets; specific side effects; and side effect severities.

For example, if you enter a fast with clinical deficiencies in various B-vitamins (e.g. B12) and electrolytes (e.g. sodium) – you may set yourself up for a faster onset of orthostatic hypotension, dehydration, fainting, neurological damage, and death – in the fasted state.

On the other hand, if you enter a fast with adequate stores of vitamins and minerals in the body, you should be able to fast for a longer duration before experiencing side effects and/or adverse reactions that are either: (1) exacerbated by or (2) attributable to – deficiencies.

Monitoring levels of vitamins and minerals before and during a fast – plus supplementing if deficiencies occur – can be an effective way to prevent and attenuate various fasting-related side effects and adverse reactions.

2. Duration of the fast

Generally speaking, prolonged water fasts tend to yield more total side effects and severe side effects – than short-term water fasts.

  • Short-term fast: Lower risk of side effects
  • Long-term fast: Greater risk of side effects

Why? Because longer fasts are associated with more: profound physiologic adaptation to the fasted state, muscle loss, fat loss, vitamin/mineral depletion – relative to shorter fasts.

A person who engages in a 30-day water fast will likely experience more total side effects and more severe side effects than if he/she only engaged in a 3-day (72 hour) water fast.

3. Supplementation (?)

Supplementation with vitamins, minerals, and amino acids – or lack thereof (i.e. zero supplementation) – while water fasting may impact the side effects that you experience as well as their severities.

In prolonged water fasts, supplementation with vitamins and/or minerals can help prevent side effects and adverse reactions attributable to fasting-related vitamin/mineral deficiencies.

While some might consider a “true water fast” to include only water (zero supplements), there isn’t really a downside associated with vitamin/mineral supplementation in proper quantities to prevent severe deficiencies.

Moreover, some individuals will utilize branched-chain amino acids (BCAAs) to minimize side effects like muscle loss during short-term fasts, however, using BCAAs for prolonged fasts is not recommended due to potential interference with: autophagy (i.e. cell repair) and ketosis (via spiking insulin).

4. Breaking the fast (Method & Rate)

Breaking a water fast improperly can cause “refeeding syndrome,” a potentially fatal condition characterized by notable shifts in fluids and electrolytes as a result of hormone and metabolic changes.

Signs of refeeding syndrome include: hyperglycemia, difficulty breathing, heart rate shifts, nerve pain or tingling, mental confusion, abnormal serum sodium concentrations, fluid retention, and muscle weakness.

The longer you’ve fasted and/or the lower your body weight – the greater your risk for refeeding syndrome.

Even if you don’t end up with refeeding syndrome, breaking a fast too quickly can cause severe post-fast side effects (stomach pain, headaches, dizziness, nausea, vomiting, diarrhea, etc.).

Breaking your fast properly by gradually introducing small quantities of liquid calories (e.g. two 8 oz cups of fruit/veggie juices or bone broths diluted with 50% water) over a period of days or a week (depending on the fast duration) can decrease risk of post-fast side effects.

Gradually reintroducing non-acidic fruits like melons, grapes, and pears and/or tolerable zero or low fiber foods like plain yogurt for several days or a week after you’ve completed your liquid calorie regimen should help to minimize risk of post-fast side effects.

Thereafter, slowly adding in select tolerable foods over a period of days or a week should help negate post-fast side effects by restoring electrolytes, hormones, and digestion back to homeostasis.

In summary: The faster you break your fast – the greater your risk of severe post-fast side effects and reactions (e.g. refeeding syndrome).  The slower you break your fast – the lower your risk of post-fast side effects.

Causes of side effects while water fasting (List)


Rapid weight loss. Rapidly losing body fat and muscle can cause side effects in some individuals.  For example, rapid body fat loss can cause Eustachian tube dysfunction.

Ketosis. Though many side effects can diminish or cease once a person has reached ketosis, some individuals experience side effects like: bad breath, poorer focus, fatigue, cramping, and electrolyte losses.

Detoxification. Although mainstream fasting champions erroneously use “detox” as a blanket explanation for every side effect fasters experience, the body can purge certain toxins like persistent organic pollutants (e.g. DDT, hexachlorobenzene, polychlorinated biphenyls, dioxins) and other lipophilic compounds as a result of fat loss – which could exacerbate or directly cause certain side effects.

Circadian dysfunction. Because meal times may help regulate normative circadian processes, it is suspected that water fasting can cause circadian dysfunction.  The circadian dysfunction that occurs during a water fast could indirectly or directly induce or exacerbate certain side effects. (R)

Hormone fluctuations.  Production of various hormones such as: melatonin, cortisol, testosterone, prolactin, LH, FSH, GH, TSH, thyroxine, and triiodothyronine may change while fasted – in part due to shifts in circadian biology.  It’s reasonable to suspect that significant hormone fluctuations could contribute to fasting-related side effects.


Hypoglycemia. Low blood sugar can cause a variety of side effects like cognitive deficits, psychiatric symptoms (anger, anxiety, depression, inattentiveness, irritability) and fatigue. (Side effects from hypoglycemia can be reduced by following a ketogenic diet prior to fasting).

Dehydration. Failure to maintain adequate hydration during a water fast can cause serious adverse reactions and exacerbate preexisting side effects. (Side effects from dehydration can be avoided by drinking enough water during the fast).

Electrolyte deficits. Failure to supplement with electrolytes during a water fast (especially if prolonged) can lead to electrolyte deficiencies and corresponding side effects. (Side effects from electrolyte deficits can be avoided by supplementing with electrolytes while fasting).

Fasting too long. The length of a water fast should be flexibly determined based upon the specific person fasting (weight, body fat, vitamin/mineral stores, health status). If a person fasts too long, he/she might: (1) reach an unhealthy weight or body fat percentage; (2) develop nutrient deficiencies; and/or (3) experience organ dysfunction – all of which could trigger serious adverse reactions.

Fasting when you should NOT fast.  Fasting is not a smart idea for everyone.  Persons with low/deficient vitamin/minerals, high stress, low body weight/fat, and untreated medical conditions – should not fast.  Those who are pregnant/breastfeeding, using certain substances (drugs, medications, supplements), or withdrawing from certain substances (drugs, medications, supplements) not fast.  (All of these scenarios increase risk of adverse reactions during a fast).

  • Deficient or low vitamin/mineral stores
  • High stress & sleep disturbances
  • Low body weight/body fat
  • Pregnant or breastfeeding
  • Substance use (drugs, medications, supplements)
  • Untreated medical conditions
  • Withdrawal (alcohol, drugs, medications)

High stress & sleep disturbances.  Although fasting can provoke an acute stress response and sleep disturbances, making a conscious effort to avoid stressful scenarios (e.g. people, places, things that stress you out) and manage stress (e.g. via relaxation) and maintain good sleep hygiene – may help decrease side effects during a water fast.

Synergism with medical conditions, substances, withdrawal. Some side effects experienced during a fast may be amplified due to physiologic synergism of the fasted state and medical conditions, substances (used while fasting), and substance withdrawal (e.g. caffeine withdrawal). (Synergistic side effects can be avoided by ensuring that you have no untreated medical conditions, aren’t using any substances, and that you’ve completed substance withdrawal prior to the fasted state).

Refeeding syndrome (*): Refeeding syndrome is defined as potentially fatal shifts in fluids and electrolytes (triggered by hormonal and metabolic changes) as a result of breaking a fast improperly.  Though it technically isn’t a side effect of water fasting – it can occur if the fast is broken improperly. (R)

My 7-Day Water Fast (Side Effects)

Back in late October 2016 I engaged in a 7-day water fast after being convinced by Dr. Thomas Seyfried (Boston College) that it would likely improve health and decrease cancer risk.

Day 1: Experienced a bit of hunger and fatigue (mental and physical) late in the day – but nothing major.  I’d fasted 24 hours before this experience, so I knew what to expect on Day 1.

Day 2: Experienced psychological stress, intense food cravings, and stronger hunger than on Day 1.  Seeing food online (e.g. ads, YouTube, etc.) amplified my cravings.  Strong cravings for McDonald’s fries (I hadn’t had McDonald’s for years at this time), ice cream, pizza (any kind), eggs, and mashed potatoes with gravy.  Energy level was lower (physically and mentally) than Day 1. Felt extremely fatigued and as though my reflexes were impaired.

Day 3: Food cravings were even more extreme than on Day 2 (which I didn’t think was possible).  I continued craving McDonald’s fries – and had food visuals popping up on my computer, phone, TV, etc. I also noticed a funny/weird taste in my mouth (almost metallic) that was difficult to describe.  Herpetic whitlow flare-up on finger – probably due to fasting-mediated changes in immune function.  Research indicates that fasting may fuel viral infections (especially herpes and influenza viruses) – and hinder certain bacterial infections. (R)

Day 4: Continued dealing with hunger (though probably not as intense as on Day 3) – and continued thinking about food.  Noticed extreme lightheadedness, hypotension, weakness, fatigue, and felt slightly faint.  Modest tongue whitening was observed along with “keto” breath.  Ketones were elevated (confirmed with KetoStix).  Felt as though I was “going to die” prior to falling asleep.

Day 5: Emotionally flat. Weak. Out of energy. Slightly disoriented. Mild shakiness upon standing (due to feeling weak). Jaw didn’t want to open to brush my teeth (also from weakness). Wasn’t hungry in the morning, but experienced food cravings in the afternoon – especially poached eggs with toast.  (These cravings were likely triggered by me smelling food in a slow-cooker). Other foods I was craving included: bacon, pizza, tacos, guacamole with tortilla chips, etc. Was not craving any fruits or veggies.

Day 6: Orthostatic hypotension. Dizziness (less than Day 5). Weakness. Fatigue. Could still hold a conversation just fine. Felt relaxed. Older childhood memories (that I never would’ve recalled without fasting) came to the forefront of my thoughts.  Had a weird dream early this morning.  Still experienced intense hunger upon seeing/smelling food. Cognition was poor for tasks beyond verbal conversation – and I didn’t feel particularly “clearheaded” like some report.

Day 7: Great sleep quality from Day 6 to Day 7. No hunger in the morning. Extremely fatigued and low on energy. Functioned alright mentally – but struggled physically.  Orthostatic hypotension episode upon getting out of bed in the morning (dropped to my knees and thought I was going to faint in the hallway). Thickest patch of whiteness on tongue due to keratin accumulation, zero food intake, and no tongue scraping.

Refeed: The refeed went extremely well.  I broke my fast with 2 servings of 8 oz fruit/veggie (Naked) juice diluted 50% with water.  Experienced burping but zero stomach pain.  Stomach rumbled a bit.  Had a mild headache after first day of refeed.  Drinking the juice increased my appetite. One serving in evening – the next 12 hours later in the morning.  Next day introduced some small servings of melons.  The following day I consumed poached eggs.  Tolerated all well – and continued ramping up calories and increasing food variety over a 1-week span. No issues.

Recap of Drew’s side effects…

  • Hunger (extreme). For me, hunger was most extreme during the first 2 to 3 days of my fast. It improved towards the end of the fast – but any sights or smells of food provoked a hunger response.  (Hunger never disappeared for me like some report).
  • Food cravings. Almost like an uncontrollable survival mechanism – my brain was flooded with strong food cravings (many of which were unpredictable) during the fast.
  • Orthostatic hypotension. This became noticeable after just a couple days fasted. (It may have been preventable with electrolyte supplementation).  Nearly fainted by the end of the fast as a result.
  • Fatigue & low energy. Physical fatigue and low energy were most apparent – and continued to worsen throughout the fast. Mental fatigue was less severe than physical fatigue.
  • White tongue, metallic taste, bad breath. The tongue whitening was from keratin accumulation. The metallic taste and (subjectively) bad breath were likely from ketosis.  I brushed my teeth twice per day during this fast.
  • Lightheadedness & dizziness. These side effects were attributable to hypotension and orthostatic hypotension. (They may have been avoidable or lessened with electrolyte supplementation).
  • Emotional flatness. I didn’t experience any crazy emotional upheavals or fluctuations in mood while fasting. Early in the fast I experienced a bit of stress and anxiety – but I noticed profound relaxation by Day 7.  It felt like my brain lacked energy to generate emotions.
  • Sleep disturbances & weird dreams. Early in the fast I experienced sleep disturbances (probably due to hunger) and extremely odd dreams. By the end of the fast, my sleep quality (as measured by the Sleep Cycle app) was nearly perfect.
  • Modest cognitive decline. I experienced a modest decline in cognitive performance during the fast (relative to a non-fasted state) – such that I was unable to accomplish cognitively-demanding, work-related tasks.

How to minimize risk of side effects while water fasting…

It may be possible to prevent or reduce the severities of certain side effects while water fasting by following the recommendations listed below.

1. Consult a medical doctor

Before water fasting, consult a medical doctor to ensure that water fasting is safe with respect to:

  • Medical history & preexisting conditions
  • Current or recent substance (e.g. medication, drug, supplement) use
  • Body composition (fat, muscle, etc.)
  • Vitamin/mineral status

A medical doctor with knowledge of water fasting may be able to help:

  • Inform you of common side effects & adverse reactions
  • Plan your water fast (e.g. determine the duration)
  • Recommend supplements to take
  • Monitor your health during the fast (e.g. intermittent check-ups)
  • Provide instruction for safely breaking the fast

2. Know when to stop fasting

Regardless of how well you’re prepared for a water fast, it’s important to know when to stop (i.e. break) the water fast.

Fasting should be immediately and safely stopped upon:

  • Adverse reactions (e.g. organ dysfunction)
  • Unbearable side effects
  • Low: body weight, body fat, muscle, BMI
  • Relapse or flare-up of a medical condition

If you’re unsure about whether you should stop your water fast – consult a medical doctor for an evaluation and his/her recommendation.

3. Health evaluation & monitoring


Prior to a water fast, it is recommended to consult a medical doctor for a thorough evaluation to ensure that there are no undiagnosed medical conditions or health abnormalities prior to your fast.

Furthermore, you may want to ensure that your body fat percentage (via measurements) and vitamin/mineral stores (via bloodwork) are sufficient – especially if you’re planning a prolonged fast.

Fasted state

During a water fast, it is recommended to monitor vitals (body temperature, pulse rate, respiration rate, and blood pressure) to ensure no significant abnormalities.

If planning a prolonged fast, it may be smart to plan intermittent check-ups with a medical professional to ensure normal: vitals; organ function; and vitamin/mineral levels.

For some individuals (such as persons planning extremely long fasts), it’s probably best to fast at a fasting clinic under the supervision of medical professionals – to minimize risk of adverse reactions.


In select cases, it may be smart to seek medical supervision to ensure that you’re breaking the fast safely – such as to avoid “refeeding syndrome” and other complications.

It may also make sense to have a doctor reevaluate your health post-fast to determine vitamin/mineral levels and rule out fasting-induced complications.

4. Hydration & electrolytes


Because “dry fasting” (i.e. fasting without water) provides no major benefits relative to water fasting and increases risk of serious adverse reactions (e.g. death) – it makes logical sense to stay hydrated during your fast.

Adequate hydration can minimize risk of dehydration and dehydration-related side effects while in the fasted state, including: dry mouth, fatigue, kidney dysfunction, dizziness, confusion, fever, tachycardia, et al.

While fasting, the kidneys purge significantly more water than in a non-fasted state – increasing risk of dehydration.

For this reason, it is recommended to consume sufficient water while in the fasted state; water needs will vary depending on the person (most people consume 1 to 3 liters per day of water).


Because the kidneys also purge significant quantities of electrolytes while fasting (sodium is most notable – followed by potassium), it makes sense to supplement with sodium, potassium, and possibly other electrolytes (e.g. magnesium, calcium, etc.) in prolonged fasts to minimize risk of, and/or severities of, side effects.

5. Vitamin supplementation (?)

In the event that: (1) you’re entering a water fast with adequate vitamin stores and (2) you’re fasting for a short-term (e.g. under 7 days) – it’s probably unnecessary to supplement with vitamins while fasting.

On the other hand, if: (1) you’re entering a water fast with vitamin deficits (this is never recommended) or (2) you’re fasting for a prolonged period (e.g. over 7 days) – you may need to supplement with highly-bioavailable vitamins to avoid deficiencies (and the corresponding adverse complications) while fasting.

Some individuals may want to take a highly bioavailable multivitamin – whereas others may only want to take select vitamins to address specific deficiencies (as detected by bloodwork) in the fasted state.

Anyone engaging in a long-term fast should get bloodwork done before a fast (to address any vitamin deficiencies) and at various intervals/checkpoints during the fast (e.g. every 30 days) to detect and address vitamin deficiencies that arise with longer-term fasts.

6. Safely break the fast

There are many methods recommended online for breaking a water fast, however, most share similar ideas:

  • Longer fasts require more time to break, shorter fasts require less time to break
  • Two servings per day of fruit juices or bone broth (8 oz diluted with 50% water) for the first 1 to 3 days post-fast may be ideal
  • Melons or pears (in limited amounts) be introduced along with liquid servings on Day 2, 3, or 4 (depending on the fast duration)
  • Yogurt (in limited amounts) may be added a day or two later (Day 3 to 5 after breaking the fast)
  • Soft boiled eggs (in small amounts) may be a good choice on Day 4 to 7 – depending on how you feel
  • Easy-to-digest foods should be used to break a fast – whereas high-fiber foods should be avoided (as these can irritate the GI tract)

I like the guide “How to Break a Fast” (WikiHow) – as it helped me successfully break my 7-day fast with zero side effects.

Eventually, more foods are introduced and the individual has safely transitioned from fasted physiology to fed homeostasis.

If done correctly, refeeding syndrome (the potentially fatal condition associated with metabolic stress upon feeding after a prolonged fast) can be avoided – as can other side effects associated with refeeding (e.g. stomach pain, diarrhea, nausea, etc.).

(While breaking a fast, preexisting vitamin/mineral supplementation should be continued to minimize odds of post-fast deficiencies).

Have you experienced side effects from water fasting?

In the event that you’ve attempted and/or completed a water fast – feel free to leave a comment documenting whether you experienced side effects or adverse reactions.

  • Which specific side effects did you experience?
  • What were the respective severities of the side effects? (Rate on a scale from 1 to 10 – with higher numbers indicating greater severity)
  • Did you use any strategies to help you manage the side effects? (Were these strategies effective?)
  • How long did it take to recover from these side effects?
  • Did you experience any permanent side effects from your fast?

To help others accurately understand why you may have experienced side effects during a water fast – provide additional details in your comment such as:

  • Duration of your water fast
  • BMI & body fat % (pre/post-fast)
  • Water intake (per day)
  • Supplements administered (e.g. vitamins and minerals)
  • Preexisting medical conditions
  • Prior or ongoing medication/substance use (e.g. alcohol, caffeine, cannabis, pharmaceutical meds, etc.)
  • Vitamin/mineral levels (pre/post fast)
  • How you ended the fast (foods consumed, quantities consumed, days it took to transition to pre-fast eating)

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